Foreign fighters are making more of a mark on the battles raging across Syria and Iraq than initially thought.

U.S. intelligence officials estimate that more than 18,000 foreign fighters now have flocked to the region -- up from about 16,000 at the start of November. An estimate by the National Counter Terrorism Center in September had put the number of foreign fighters at more than 15,000.

The number of Western passport holders joining the fight also has grown, to at least 3,000. Earlier estimates had put the number of Westerners fighting in Iraq and Syria at about 2,700.

Officials caution the higher estimates do not mean, however, there are necessarily more fighters on the battlefield.

“Countries are scrutinizing suspect travel more closely,” an intelligence official told VOA on the condition of anonymity. “So, more foreign fighters are being captured in the count.”

Just recently, the government in Jakarta raised its estimate of Indonesian fighters in Syria from 300 to 350, telling VOA's Indonesian Service better information on the use of aliases and on Indonesians living abroad was a key factor.

"The problem is their IDs are not identical with their aliases. Therefore, we have to crosscheck them for validation," said Wawan Purwanto, an expert with Indonesia’s Anti-Terrorism Management Agency [BNPT].

U.S. intelligence officials say most of the foreign fighters heading to Iraq or Syria seem to be intent on joining with the Islamic State, although many are still fighting with the al-Qaida-affiliated Nusra Front or with other groups.

There also is hope that the flow of foreign fighters to the region will begin to subside.

“The Europeans and other allies are taking steps to stem the flow of their citizens to Syria and Iraq, while the Turks are trying harder to keep their borders from being exploited by jihadists,” the U.S. intelligence official said. “It could be a while for the dampening effect of these measures to start showing up in the foreign fighter intelligence estimates.”